Prior art references relate to structures adapted to expand the radial projection of the device after the device has been inserted into a cavity. Three groups of prior art are outlined by structural similarities in their expansion characteristics. A fourth group relates to other expansion characteristics.
Axial contraction is used to produce radial expansion once a device has been inserted into a cavity. Wigam (U.S. Pat. No. 3,505,921) and Talan (U.S. Pat. No. 4,309,136) disclose construction fastening devices. Fischer et al. (U.S. Pat. No. 3,779,239) and Rublenik (SU 1,386,182-A) relate to elongated fasteners that employ radial expansion elements at the distal end of the device. These fasteners are intended to secure fractured portions of bone tissue. Kuslich (U.S. Pat. No. 5,059,193) relates to a spinal implant for use between vertebrae. Tansey (U.S. Pat. No. No. 4,681,590) relates to a structure having metal strips secured between an upper plate and a nut. The nut is mounted on a screw and constrained against rotation so that rotating the screw reduces the axial separation of the nut and upper plate causing the metal strips to expand radially. Tansey also relates to a femoral stem prosthesis.
Oblique contact has been used between moving elements to expand the radial projection of devices that have been inserted into a cavity. One common example of this type of fastener is from Aginsky (U.S. Pat. No. 4,091,806). In this mechanism, a central shaft is displaced relative to an outer concentric shaft. The central shaft includes a wedge that obliquely contacts a longitudinally slotted portion of the outer concentric shaft. The oblique contact translates the axial force on the wedge into a radial force that expands the outer concentric shaft radially. Prior art having multiple elements actuate the elements such that the elements cannot be expanded independently to adapt to the contours of the cavity in which they are placed.
Pivotal connections have been used to expand the radial projection of devices once they are inserted into a cavity. Prior art relates to elements pivotally connected to the device that are contacted by an axially displaceable element. The axial force at a distance from the pivotal connection creates a torque that rotates the pivotally connected elements into a new position that has a greater radial diameter. Some of these references disclose mechanisms that also use oblique contact to provide the necessary torque. As examples of this type of mechanism, see Avila (U.S. Pat. No. 3,986,504), Davis (U.S. Pat. No. 5,057,103), Dobelle (U.S. Pat. Nos. 2,685,877 and 3,024,785), and Firer (SU 1,524,880A). None of the references in this group relate to means for coordinated self-seeking conforming of elements. Aginsky (U.S. Pat. Nos. 4,204,531 and 4,227,518) relate to use of pivotal connections in a different structure. A pivot point is movably mounted in a longitudinal slot. The pivot point is pivotally coupled to two legs that are pivotally coupled at their other ends to two sections of the outer sheath. When the outer sheath is displaced axially, the pivot point is constrained by the slot and the sections of the outer sheath are rotated radially by the legs. This structure does not allow coordinated self-seeking conforming actuation of the two outer sheath sections.
Bolesky (U.S. Pat. No. 4,275,717) and Street (U.S. Pat. No. 3,216,414) relate to elements that are biased to expand radially. These elements are elastically constrained by a ring or cap that is axially displaced once the device is inserted so that the biased elements can resume a radially expansive position. Erlich-DeGuemp (FR 2,387,638) relates to a device that uses the bone tissue surface to provide an oblique contact for radial expansion. Muhlbayer (DT 1,075,793) relates to use of a rotating central shaft to translate a band which has three pinned elements that are allowed to rotate radially. The pinned elements can rotate freely but are not driven by a mechanism and do not provide a means to engage the three rotating elements in a coordinated self-seeking conforming manner.
A review of the product literature shows adaptations of mechanism similar to Livingston (U.S. Pat. Nos. 2,699,774 and 2,490,364), Fisher (U.S. Pat. No. 3,805,775) and Flander (U.S. Pat. No. 3,708,883). An Alta Modular Trauma System product (Howmedica, Rutherford, N.J.) uses a slotted sleeve, wedge shaped inner mandrel and translation of the mandrel in the sleeve to increase radial diameter. Other companies are generally introducing unicortical fasteners (engages only one bony cortex) consisting of a slotted externally threaded hollow cylinder with a threaded inner mandrel that when rotated expands the radial diameter of the outer cylinder. One example of this type of device is the Sargon Implant system (Sargon Enterprises, Inc., Beverly Hills, Calif.).
Bone implants have been used to solve health care problems of orthopedic and maxillofacial reconstruction, prosthesis fixation, drug delivery and fracture stabilization. Heretofore, bone and cartilage (hard tissue) implants were fastened with screw threads, interference fits, uniformly expanding mechanisms and cement. The majority of these devices and associated techniques provide poor initial fixation, and following bone formation around the device, provide good fixation but often for only a limited period of time. Implant removals are frequently performed following failure of the bone-implant interface and clinical loosening of the device.
The principal cause for implant failure in hard tissue is the separation of bone from the surface of the implant. Bone resorption about an implant is induced by micromotion of the device relative to the surrounding hard tissue, adverse tissue reaction to the implant material, or tissue necrosis due to drill heating and mechanical stress concentrations. Micromotion is often due to poor initial stabilization of a threaded, interference fit or cemented device.
Bone and cartilage are tissues with viscoelastic material properties. Their modulus of elasticity and ultimate strength are much less than the metal and ceramic materials used for hard tissue implants. This mismatch in material properties is a factor in device-tissue interfacial micromotion, interface stress concentration and implant loosening. This problem is compounded by bone's range of morphology and material properties.
The bone organ contains two distinct types of bone tissue. Cortical bone is the hard structural bone that forms the outer shell of the skeleton. Cancellous bone is contained within cortical bone and makes up a significant portion of the volume of most bones. Cancellous bone is porous, trabecular in structure, highly vascular, filled with cellular elements and undergoes active remodeling (formation and resorption).
Heretofore bone implants placed transverse to the long axis of bone penetrated a short segment of the cortical bone shell and had a significant portion of their surface adjacent to cancerous bone. These devices relied on both the cortical and cancerous bone for initial and long-term stability. Implant dependence on porous low strength cancellous bone and limited cortical bone contact causes poor device stability.
Heretofore bone implants that were placed along the long central axis of bone were designed to occupy much of the cancerous bone space and contact the endosteal surface (inner surface) of cortical bone. The irregular morphology of the endosteal surface caused point-contacts and concentrated loads between these devices and cortical bone. Initial implant fixation was achieved by interference fit at these points of contact.
High concentrated loads and stress-shielding between point-contacts can cause resorption of bone and implant loosening. Long-term implant fixation is dependent on cancellous bone growth around the surface of the implant where it is not contacting the endosteal surface of cortical bone.
Cements are commonly used to fill the void between the endosteal surface of cortical bone and the implant. Though commonly used, cements can cause adverse tissue reactions and complicate the load characteristics of the bone-implant interface by adding a third material with its unique material properties. Uncemented devices are being introduced for hip and knee prostheses. Some of these devices are contoured or require the endosteal surface of the cortical bone to be machined to increase the implant-to-cortical bone surface area. Contouring the device surface increases its cost and may require patient imaging and bone shaping. Machining bone removes healthy tissue and weakens the structural strength of the organ. Additionally, local heating of bone during machining can cause its death.
Hand-tool reaming, press-fit installation and cementing require skill. These procedures are sources of surgical variance and potential prostheses failure. The combined issues of device design, device-bone interface failure, micromotion, stress-concentration, stress-shielding material property differences and surgical variance limit the useful lifetime of many prostheses to 5 to 20 years.
The present invention addresses these problems in the prior art and provides devices having coordinated self-seeking conforming members, where a known relationship exists between the pressure exerted on the surrounding material and the mechanism engagement torque or force, and having the ability to contour to an irregular defect.